1. Field of the Invention
The present invention relates to a doping apparatus and a method of doping used for the manufacture of semiconductor integrated circuits and the like. More particularly, the present invention relates to an ion doping apparatus and a method of doping having a configuration preferable for processing substrates having large areas, wherein ion beams are radiated to a semiconductor material composed of amorphous components partly or entirely or to a substantially intrinsic polycrystalline semiconductor material to supply impurities to the semiconductor, material.
2. Description of Related Art
Methods of forming p-type and n-type impurity regions in a semiconductor during the manufacture of semiconductor integrated circuits and the like are known in which ions of impurities that produce n and p conductivity types (n-type impurities and p-type impurities) are radiated and implanted by accelerating them by a high voltage. Especially, a method of separating mass and charge ratio of ions is referred to as xe2x80x9cion implantationxe2x80x9d and are widely used for the manufacture of semiconductor integrated circuits.
Another method is known in which plasma including n-type and p-type impurities is produced and ions in the plasma are accelerated by a high voltage to be implanted in a semiconductor as an ion current. This method is referred to as xe2x80x9cion dopingxe2x80x9d or xe2x80x9cplasma dopingxe2x80x9d.
The structure of a doping apparatus utilizing ion doping is simpler than that of a doping apparatus utilizing ion implantation. For example, to implant boron as p-type impurities, plasma is produced in a gas of diborane (B2H6) which is a boron compound by means of RF discharge or the like and a high voltage is applied to the plasma thereto to extract ions including boron which are in turn radiated into a semiconductor. Since gas-phase discharge is performed to produce plasma, the degree of vacuum inside the doping apparatus is relatively high.
Presently, an ion doping apparatus is frequently used to add impurities uniformly to a substrate having a relatively large area. This is because an ion beam to cover a large area can be relatively easily obtained in an ion doping apparatus which does not perform separation on a mass basis. On the other hand, for an ion implantation apparatus which must perform separation on a mass basis, it is difficult to increase the area of a beam while maintaining the uniformity of the ion. Therefore, an ion implantation apparatus is unsuitable for a substrate having a large area.
Recently, studies are active on the reduction of the temperature for semiconductor device processing. This is largely because of the fact that a necessity has arisen to form semiconductor devices on inexpensive insulated substrates made of glass and the like. Other reasons include needs associated with the trend toward microscopic devices and multi-layer devices.
Insulated substrates made of glass or the like have various advantages compared to silica substrates which have been used in processing at high temperatures in that they are easy to process, easy to form with a large surface area, inexpensive, and so on. However, as a matter of fact, the trend toward substrates having larger areas has also resulted in various difficulties to be technically overcome including a need for developing apparatuses having characteristics different from those suitable for conventional processes at high temperatures.
Ion implantation is disadvantageous for the manufacture of active matrix type liquid crystal displays and like wherein substrates having a large area must be processed, and ion doping is under research and development in an intention to cover such a disadvantage.
FIGS. 1 and 2 schematically illustrate a conventional ion doping apparatus. FIG. 1 schematically illustrates an ion source and an ion accelerator mainly. FIG. 2 illustrates the structure of the ion doping apparatus as a whole. The description will first proceed with reference to FIG. 1. Ions are generated in a plasma space 4.
Specifically, radio-frequency power is applied between an electrode 3 and a mesh electrode 6 by a radio-frequency power supply 1 and a matching box 2 to generate plasma in the plasma space 4 under a reduced pressure. Hydrogen or the like is introduced at the initial stage of plasma generation, and diborane and phosphine (PH3) which are doping gases are introduced after the plasma is stabilized.
The electrode 3 and the outer wall of the chamber (at the same potential as that of the mesh electrode 6) are insulated from each other by an insulator 5. An ion current is extracted from the plasma thus generated by an extraction electrode 10 and an extraction power supply 8. The ion current thus extracted is shaped by a suppressor grid 11 and a suppressor power supply 9 and thereafter accelerated into required energy by an acceleration electrode 12 and an acceleration power supply 7.
FIG. 2(A) will now be described. The ion doping apparatus is generally comprised of an ion source/accelerator 13, a doping chamber 15, a power supply device 14, a gas box 19, and an exhaust device 20. In FIG. 2, the ion source/accelerator as in FIG. 1 arranged horizontally. That is, in FIG. 2, the ion current flows to the right (downward in FIG. 1). The power supply device 14 mainly consists of power supplies used for generation and acceleration of ions and includes the radio-frequency power supply 1, matching box 2, acceleration power supply 7, extraction power supply 8, and suppressor power supply 9.
A substrate holder 17 is provided in the doping chamber 15, and a material 16 to be doped is placed thereon. In general, the substrate holder is designed such that it can be rotated about an axis in parallel with the ion current. The air in the ion source/accelerator 13 and the doping chamber 15 is exhausted by the exhaust device 20. The air in the ion source/accelerator 13 and the doping chamber 15 may be exhausted by separate exhaust devices.
A doping gas is delivered from the gas box 19 to the doping chamber 15 through a gas line 18. While a gas intake port is provided between the ion source/accelerator 13 and the material 16 to be doped in the apparatus shown in FIG. 2(A), it may be provided in the vicinity of the plasma space 4 of the ion source. The doping gas is generally used by diluting it with hydrogen or the like.
In the conventional ion doping apparatus, the area of a substrate (material to be doped) has been equal to or smaller than the sectional area of the plasma space 4 in the ion source 13. This is a requirement to be satisfied to achieve uniform doping. FIG. 2(B) illustrates a section which is perpendicular to the ion current. Specifically, the ion source/accelerator 13 has a size represented by L1 and L2, and the doping chamber 15 and a material 17 to be doped are sized such that they can be contained therein. The dimensions L1 and L2 are about the same.
Therefore, the size of the plasma space 4 must be increased with the size of the substrate. Further, plasma must have two-dimensional uniformity. However, it is difficult to increase the size of the plasma space infinitely. The reason is that this makes the generation of plasma nonuniform. This is primarily attributable to the fact that the mean free path of molecules becomes sufficiently smaller than the section of the plasma space. It is therefore difficult to make the length of one side of the plasma space equal to or greater than 0.6 m.
The present invention is characterized in that an ion current is shaped to have a linear or rectangular section and in that a material to be doped is moved perpendicularly to the longitudinal direction of the ion current (i.e., in the direction of the shorter dimension of the ion current). As a result, plasma is required to be uniform only in the longitudinal direction, and this makes it possible to process a substrate having a large area. What is to be considered is only the uniformity of plasma in the longitudinal direction and not two-dimensional uniformity because irradiation with ions is carried out by scanning in any part of the material to be doped.
According to the present invention, in principle, while the length of one side of a substrate is limited by the length of plasma, there is no factor limiting the length of another side of the substrate other than the size of the doping chamber. It is easy to generate plasma whose uniformity is maintained for about two meters in the longitudinal direction thereof if the width of the discharge space is sufficiently small. It goes without saying that the width of the ion beam is on the order of centimeters.
Therefore, such a linear ion doping apparatus is suitable for processing a substrate having a large area and processing a multiplicity of substrates simultaneously. For example, it can relatively easily dope substrates of sizes up to 2 mxc3x97x m where x is determined by the size of the doping apparatus.
FIG. 3(A) illustrates the conception of the present invention. An ion doping apparatus according to the present invention comprises an ion source/accelerator 13, a doping chamber 15, a power supply device 14, a gas box 19, and an exhaust device 20 as in the prior art. Unlike the prior art, however, the ion source/accelerator 13 generates an ion current having a linear or rectangular section. Further, a substrate holder 17 includes a mechanism which moves during doping. The longitudinal direction of the ion current is a direction perpendicular to the plane of the drawing.
In the ion doping apparatus according to the present invention, the shape of a substrate (material to be doped) that can be processed has no relationship with the sectional shape of a plasma space 4 in the ion source 13. However, the length of one of the shorter sides of the substrates must be equal to or less than the length of the plasma space 4 in the longitudinal direction thereof. There is no factor that limits the size of another side of the substrate other than the size of the doping chamber.
FIG. 3(B) illustrates a section perpendicular to the ion current. Specifically, the shape of the ion source/accelerator 13 (L1xc3x97L2) is not limited by the shapes of the doping chamber 15 and a material 17 to be doped. Since the ion current has a linear or rectangular sectional shape, L1 less than L2 (=the longer dimension of the section of the ion current).
The statement that an ion current is required to be uniform only in the direction of the longer dimension and not in the direction of the shorter dimension thereof implies that no problem arises even if there is distribution of ionic strength and ionic species in the direction of the shorter dimension of the ion current. This is advantageous in removing certain light ions (e.g., H+ and H2+) from the ion current. It has been necessary to exert a magnetic action on an ion current to separate ions therein, which has inevitably affected the distribution of heavy ions which have been required.
With conventional ion doping apparatuses in which two-dimensional uniformity has been required, it is substantially impossible to separate ions. According to the present invention, however, it is easy to perform separation as shown in a second embodiment thereof.
The fact that an ion current is required to be uniform only in the direction of the longer dimension and not in the direction of the shorter dimension thereof is advantageous from the viewpoint of the structure of an electrode for accelerating and decelerating the ion current. A mesh-like or porous electrode has been frequently used in conventional ion doping apparatuses. However, in the case of such an electrode, since a part of ions collide with the main body of the electrode, deterioration of the electrode or splashing and sputtering of substances that form the electrode can be a problem.
On the contrary, according to the present invention, the above-described problem is solved because an electrode having a simple configuration is provided in a position apart from an ion current as shown in a first embodiment.
Known conventional semiconductor manufacturing techniques include ion implantation which involves a known technique for scanning an ion current across a fixed substrate by electromagnetically deflecting the same. However, such a method is unsuitable for doping ions having various mass-to-charge ratios as in the present invention, and it is preferable to move the substrate with the ion current fixed as carried out according to the present invention.
The reason is that, according to the technique for electromagnetically deflecting an ion current, light ions are much easily deflected than heavy ions and therefore can not be scanned uniformly. Since a difference of only one in mass numbers can cause distribution, it is not preferable to apply this technique to ion doping techniques to which the present invention is directed. The use of such a technique for electromagnetic deflection is limited to doping of only one ion species.
An ion doping apparatus according to the present invention may be added with an ion focusing apparatus and an ion mass separating apparatus which are well known in the prior ion-related art.
In a linear ion doping technique like the present invention, the feature of easy ion mass separation can result in an advantage also in a subsequent annealing process. In general, when ion doping is performed, the incidence of ions upon the substance being irradiated can result in damage to a lattice of atoms of the substance under irradiation, transition of a lattice into an amorphous state, and the like. Further, it is not possible to cause the dopant to work as a carrier by simply implanting it in a semiconductor material. Several steps are required to follow doping in order to solve these problems.
The most popular method employed in such steps is thermal annealing or optical annealing. Dopant can be combined with a lattice of a semiconductor material by performing such annealing. In the case of optical annealing, however, light must reach a location where damage to a lattice has occurred or the like as described above.
It is considerably common to perform another step of adding hydrogen to eliminate levels (uncombine arms) which have survived the above-described annealing. Such a step is referred to as xe2x80x9chydrogenationxe2x80x9d. Hydrogen easily enters in a semiconductor material at a temperature on the order of 350xc2x0 C. and eliminates the levels as described above.
In any case, the inclusion of such steps after doping is not preferable from the viewpoint of cost and throughput because it increases the number of steps. By performing thermal annealing and hydrogenation simultaneously with doping or performing a part of those steps during doping, it is possible to eliminate separate steps for annealing and hydrogenation, to reduce processing time or to decrease the processing temperature.
It is relatively easy to add hydrogen and dopant in a semiconductor material simultaneously. Specifically, doping may be performed by diluting dopant with hydrogen and ionizing it together with hydrogen. For example, if ion implantation is carried out by the doping apparatus as shown in FIGS. 1 and 2 using phosphine (PH3) diluted with hydrogen, hydrogen ions (e.g., H2+ and H+) will be implanted along with ions including phosphorus (e.g., PH3+ and PH2+).
However, since hydrogen is too light and easily accelerated compared to ions including dopant such as phosphorus and boron, it penetrates too deep in the substrate. On the other hand, ions including dopant stay in a relatively shallow region. Therefore, in order for hydrogen to correct defects caused by dopant, hydrogen must be moved by means of thermal annealing or the like.
Meanwhile, the use of a linear ion beam makes it possible to irradiate a substrate with only desired ions by providing a mass separator on the way of an ion current as described above. A new doping method as described below can be derived from such an idea. That is a doping method wherein ions having different mass are separated and then accelerated at different voltages, and resultant beams are radiated to a semiconductor material to implant those ions to substantially the same depth.
For example, separation is performed to obtain ions mainly composed of hydrogen (light ions) and ions including dopant (heavy ions), and only the latter is accelerated to make the depths of penetration of the light and heavy ions substantially the same. Thus, the presence of the light ions makes it possible to simultaneously perform a part of or all of an annealing step and a hydrogenation step on the dopant.
Specifically, the speed of incidence of a hydrogen ion beam upon a semiconductor material is made close to the speed of incidence of an ion beam containing the dopant upon the semiconductor material. As a result, the distribution of hydrogen in the semiconductor film is made close to the distribution of the dopant. At this time, the dopant is immediately activated by incidence energy of ions (which is converted into thermal energy as a result of collision) and the presence of hydrogen. This effect allows a subsequent dopant activation step to be eliminated.
The depth of penetration of each ion beam may be adjusted by changing its angle of incidence. The smaller the angle of incidence, the smaller the depth of penetration. The angle of incidence may be changed by magnetic and electrical effects. Ions can not enter a substrate and are reflected therefrom if the angle of incidence is too small. An angle of incidence of 40xc2x0 or more will be sufficient.
For the above-described purpose, a mass separator may be provided between an ion beam generator and an accelerator. Further, mass separation can be performed on an ion beam using an apparatus which applies a magnetic field in parallel with the longitudinal direction of the ion beam.
Ion implantation into a semiconductor material may be carried out by implanting ions including dopants first and implanting ions mainly composed of hydrogen thereafter or may be performed in the reversed order.
It will be advantageous to provide an ion doping apparatus and a laser annealing apparatus utilizing a linear laser beam according to the present invention in the same chamber. Specifically, it is much more advantageous to combining them into a single apparatus than providing them as separate apparatuses considering the fact that the present invention is characterized by a step of doping a substrate while scanning it with a linear ion current; a laser annealing process utilizing a linear laser beam according to another aspect of the invention needs a similar mechanism to be implemented; and steps utilizing those apparatuses are performed consecutively.
For example, Japanese unexamined patent publication (KOKAI) No. H7-283151 discloses a multi-chamber vacuum processing apparatus including an ion doping chamber and a laser annealing chamber. The idea of integrating an ion doping chamber and a laser annealing chamber has not been adopted in conventional ion doping apparatuses which have been based on irradiation using an ion current having a planar section at a time and which have sometimes required a substrate to be rotated.
However, according to the present invention wherein an ion doping apparatus performs doping while moving a substrate with a transport mechanism similar to that of a linear laser annealing apparatus, there is no need for providing an ion doping chamber and a laser annealing chamber separately, and it is rather advantageous to integrate them from the viewpoint of productivity on a mass production basis. Specifically, an arrangement may be made wherein the longitudinal direction of a section of an ion current is in parallel with the longitudinal direction of a section of a laser beam and wherein a substrate is moved between them perpendicularly to the above-mentioned directions. This makes it possible to perform an ion doping step and a laser annealing step consecutively.
The combination of a linear ion processing apparatus with a linear laser annealing apparatus has an advantage, in addition to the advantage of reducing the number of steps by performing the two steps simultaneously, in that the possibility of contamination of a substrate is reduced.
Further, the use of an ion doping apparatus according to the present invention allows a doping process having features as described below. A first method of doping according to the present invention comprises the steps of generating a linear ion beam, separating the ion beam into at least two ion beams through mass separation on the ion beam, accelerating the ion beams by different voltages, and radiating the ion beams to a substrate at different angles.
A second method of doping according to the present invention is characterized in that it comprises the steps of generating a linear ion beam, performing mass separation on the ion beam to obtain at least two kinds of ion beams, accelerating one of the ion beams by an acceleration voltage different from that for the other, and radiating the at least two ion beams to a substrate while moving the substrate in a direction substantially perpendicular to the linear direction 6f the linearly processed ion beams.
A third method of doping according to the present invention is characterized in that it comprises the steps of generating a linear ion beam including hydrogen, separating the ion beam on a mass basis into an ion beam mainly composed of hydrogen and another ion beam, applying energy and incident angles to the ion beam mainly composed of hydrogen and the other such that the depths of penetration of those ion beams into a substrate substantially equal each other, and radiating the ion beams to the substrate while moving the substrate in a direction substantially perpendicular to the linear direction of the linearly processed ion beams.